1. Field of the Invention
The present invention relates to the coating of orthopedic replacements such as intramedullary nails, screws, hip replacements, etc., with hydrogels and other biocompatible/biodegradable materials which expand in the presence of liquids. Expansion of the outer coating causes the replacement to be fixed into position once inserted into the intramedullary bone cavity. The same principle applies in the case of similarly coated screws and nails, which are thus able to maintain their mechanical hold where they are inserted in intracortical holes, and improve the function of normal spiral screws. Coating can be done by direct polymerization of the monomer by immersion in the case of coatings made with linear polymers. The degree of pressure exercised after expansion is controlled by the degree of cross-linking of the polymeric network, by the thickness of the coating, and by the presence of organic or inorganic inclusions in the coating.
2. Description of Related Art
Current treatments for bone fractures involve the use of stiff plates which lend mechanical support to the join until the tissues have had time to heal. The plates are fixed to the bone by spiral screws screwed into the bone itself (Amis et al.: Fatigue fracture of a femoral sliding compression screw-plate device after bone union. Biomaterials, Vol. 8, 1987). As the plate is stiffer than the bone, it bears most of the strain inflicted on them, whereby the bone is actually protected from any strain. This protection results in anomalous regrowth of the natural tissue and prevents the early mending of the bone parts because of the formation of callus during the process of reconnection (Szivek et al.: A study of bone remodelling using metal-polymer laminates. J. of Biomedical Materials Research, Vol. 15, 853-865, 1981). Over time, the extreme stiffness of the plate can cause atrophy and osteoporosis.
Another problem arises from the type of fastener used to fix the plate to the bone. These are usually metal screws driven directly into the bone, and often cause local trauma by tearing the tissues holding the screw. This often causes inflammation and further problems connected with the healing of the tear.
The most widely used cements are based on acrylates, e.g., PMMA, in the monomeric or mixed monomeric-polymeric phase which are then polymerized in vivo (M. F. Refojo: Materials for use in the eye, in Polymers in Medicine and Surgery, R. L. Kronenthal et al., eds. Plenum Press, New York, Vol. 8, page 313, 1975). Direct polymerization gives rise to a series of disadvantages essentially linked to the difficulty of controlling the reaction from the outside (W. Petty: Methyl methacrylate concentrations in tissues adjacent to bone cement. J. of Biomedical Materials Res., Vol. 14, 427-434, 1980). The exothermic character of the polymerization reaction causes the formation of hot spots reaching unbearably high temperatures for the surrounding tissues, which consequently become degraded (Kliment et al.: Use of spongy hydron in plastic surgery. J. Biomed. Mater. Res., Vol. 2, 237, 1968). In order to avoid this degradation, the reaction is made as mild as possible, but this in turn decreases the degree of conversion with a consequent increase in the percentage of unreacted products which cannot be eliminated (Willert et al.: Measurements of the quantity of monomer leaching out of acrylic bone cement into the surrounding tissues during the process of polymerization. Biomedical Applications Of Polymers, H. P. Gregor, ed., Plenum Press, 1975). The absorption of unreacted acrylic monomers is a highly toxic phenomenon, and can lead to very serious consequences (Silvestre et al.: Failure of acrylic bone cements under triaxial stresses. J. of Materials Science, Vol. 25, 1050-1057, 1990).
Furthermore, in the last few years, medical research has disclosed that the pathology of articular arthrosis is also occurring in younger patients (S. Spainer: Histology and Pathology of Total Joint Replacement, in Total Joint Replacement, chapt. 7, pp. 61-74, William Petty, ed., W. B. Saunders Company, 1991). This, coupled with the extension of the average life, often requires more than one operation. The actual technique for the hip prosthesis cementation does not permit, because of the irreversibility of the polymerization, easy re-operation of the patient.
Hydrogels are a broad class of polymeric materials which swell extensively, but do not dissolve in water. They include many natural materials of both plant and animal origin. As a result of the similarities between synthetic and natural hydrogels, these gels have been used in a wide variety and growing number of biomedical applications such as opthamology (M. F. Refojo, "Materials for Use in the Eye", in Polymers in Medicine and Surgery, R. L. Krunenenthal et al., eds., Plenum Press, New York and London, Vol. 8, 313, 1975), plastic surgery (Kliment et al., "Use of Spongy Hydron in Plastic Surgery", J. Biomed. Mater Res., Vol. 2, 237, 1968), orthopedics (Migliaresi et al., "Hydrogels for Artificial Tendons," in Hydrogels in Medicine and Pharmacy, Vol. III, chapt. 4, page 83, N. A. Peppas, ed., CRC Press, 1987), pharmacy (N. A. Peppas, "Release of Bioactive Agents from Swellable Polymers: Theory and Experiments", in Recent Advances in Drug Delivery Systems, Anderson et al., eds., Plenum Press, New York, 279, 1984), as well as medical devices (Kocvara et al., "Gel-Fabric Prostheses of the Ureter," J. Biomed. Res, Vol. 1, 325, 1967) and other related applications (Rather et al., "Hydrogels for Medical and Related Application", J. Andrate, ed., ACS Symposium, Washington, D.C., chapt. 1, 1976); N. A. Peppas, "Other Biomedical Applications of Hydrogels," in Hydrogels in Medicine and Pharmacy, Vol. III, chapt 9, page 177, N. A. Peppas, ed., CRC Press, 1987).
Hydrogels are three-dimensional polymeric networks held together by crosslinks of either weak cohesive forces, hydrogen bonds, or ionic bonds. These networks imbibe large quantities of water (or organic liquids) without dissolution.
The ability of natural tissues to grow into the hydrogel matrices makes them very attractive for biomedical uses (P. A. Davis, PhD Thesis, "A Biodegradable Artificial Composite Tendon Prosthesis", University of Connecticut, 1990, Chapt. 2, pages 46-68; Kobelar et al.: "Experimental Implantation of Hydrogels into Bone", J. Biomed. Mater. Res., Vol. 22, 751, 1988). This and other attributes, such as permeability to small molecules, e.g., metabolites, a soft consistency, and a low interfacial tension between the gel and aqueous solutions are some of the important properties which have generated interest in hydrogels as useful biomaterials. Moreover, the ease of purification, adjustable mechanical properties, and high equilibrium water content, along with their sterilizability, makes these materials ideal for biomedical use.
The simultaneous presence of hydrophilic and hydrophobic groups in the complex structure of the polymeric network is responsible for the expansion of these materials in the presence of water or other polar solvents. When wet, hydrogels have a soft consistency similar to that of natural soft tissues (Hoffman A. S.: Hydrogels--a broad class of biomaterials, in Polymers in Medicine and Surgery, R. L. Kronenthal et al., eds., Plenum Press, New York, 1975). The enormous interest in these materials is mainly due to their high biocompatibility (Korbelar et al.: Experimental implantation of hydrogel into the bone. J. of Biomedical Materials Research, Vol. 22, 751-762, 1988). Initially used to make contact lenses, hydrogels are today applied in various areas of medicine and pharmacy.